1. Technical Field
The present invention relates to a double side grinding apparatus suitable for grinding both opposing surfaces of a workpiece simultaneously, or a double side polishing apparatus suitable for polishing both sides of a work simultaneously, where flatness and parallelism of both surfaces are required.
2. Background Art
Applications of semiconductor silicon wafers, optics, compact disks, minidisks and the like require their surfaces to be strictly flat and parallel to each other, and the requirement is increasing in strictness every year. Conventionally, a double side grinding apparatus is known as an apparatus capable of grinding and finishing both surfaces of a workpiece simultaneously with high efficiency, providing flatness and parallelism to both surfaces. The variation in final accuracy of a workpiece that is machined by using this double side grinding apparatus is usually a few .mu.m in flatness as well as in parallelism.
FIG. 1 shows a front elevation of one example of a conventional horizontal double side grinding apparatus, and FIG. 2 shows its profile. The double side grinding apparatus shown in FIGS. 1 and 2 is so arranged that the open sides of two cupped-type grinding wheels 1a and 1b, each having a flat grinding end face, are set facing each other and move in a manner permitting simultaneous contact with both sides of a workpiece 2 to grind them.
In the double side grinding apparatus shown in FIGS. 1 and 2, main spindles are given relative rotational movement by spindle motors, and the cupped-type grinding wheels 1a and 1b are fitted detachably at the ends of two opposite main spindles 3a and 3b having the same rotational axis. Thus, the grinding surfaces of the cupped-type grinding wheels 1a and 1b face each other. Each of the main spindles 3a and 3b is retained and restrained by a sliding or ball-and-roller guide 4. The guide 4 is fixed on a bed 8, and in a horizontal double side grinding apparatus, it is generally located underneath the main spindles 3a and 3b, with two guides disposed in parallel to the rotational axis of the main spindles 3a and 3b.
The rotating force of the main spindles 3a and 3b is transmitted by spindle motors 5 disposed above the main spindles 3a and 3b via driving belts. In addition, while FIG. 1 shows a horizontal double side grinding apparatus, a vertical double side grinding apparatus has a similar structure rearranged in the vertical direction.
The above-described conventional double side grinding apparatus has ball-and-roller bearings used for rotating the main spindles. In order to ensure the rotating accuracy of such spindles, angular contact ball bearings are placed opposite to each other and pressure is preloaded, or conical bearings are used for ensuring rigidity, but these means usually have limits, a tolerance of 1 .mu.m in accuracy and 20 kgf/.mu.m in rigidity.
Further, in the above-mentioned conventional double side grinding apparatus, sliding guideways are mostly used for the guides in consideration of damping, though, in the case of achieving minute movement at a positioning tolerance of 1 .mu.m or less, such guides are subject to sticking attributable to pitching errors in the grinding planes.
Furthermore, in the above-mentioned conventional double side grinding apparatus, as shown in FIG. 2, the distances between a working point 7 in the grinding plane of the cupped-type grinding wheels 1a and 1b and the guide way 4, and between the working point 7 and a motion center 6, are spaced apart from each other, and the greater the diameter of the main spindles 3a and 3b, the wider these distances, so that this tendency becomes increasingly conspicuous as the size of the work enlarges. However, the wider distances become between the working point 7 and the guideway 4, and between the working point 7 and the motion center 6, the more difficult it becomes to ensure sufficient rigidity, and the problem that the measuring inaccuracies caused by the Abbe's error occur.
On the other hand, recently, for example, for optical glass, or semiconductor silicon wafers, while the required accuracy of machined planes has been increasing in strictness every year, the grinding apparatus is inevitably made larger by the increased size of silicon wafers and the like. In order to ensure the accuracy of flatness to submicrons and that of surface roughness to little over 10 nm, for the finished state of workpieces by grinding with a double side apparatus, it is necessary to achieve a sufficient level of rigidity against the reaction force at the time of machining as well as a positioning tolerance of 10 to 100 nm.
It is therefore an object of the present invention to provide a double side grinding apparatus to solve or reduce the aforesaid problem, and, more specifically, to provide a double side grinding apparatus and a double side polishing apparatus capable of enhancing the accuracy, ensuring rigidity against the reaction force at the time of machining as well as reducing pitching errors.